1. Programming in R Packages, functions, and apply

2. Functions, Packages, and apply() In this session, we will learn –What are functions. –A brief introduction into user defined functions. What is a package and how to download/install packages Finally, use the apply function to execute a function on a data frame.

3. Functions Most fundamental actions in R are carried out by functions. Examples we have seen –head() –print() –read.csv() –read.table()

4. Functions A very important function is help() –help(mean) –Opens an external webpage that describes the function mean. The mean function: a<- c(1,2,3,4) mean(a)

5. Functions There are many functions already available in base R. -Basic functions -Websites with useful lists of functions NamePurpose min(x)Minimum sd(x)Sample standard deviation round(x, digit)Rounds a number to the specified digits. sum(X)Sums up values

6. Functions Users can create custom functions. Think of functions like Procs and collections of functions as macros. We will create a few functions.

7. Packages People write several specialized functions and the “package” them together. The packages are distributed on the CRAN website and can be accessed directly in R.

8. Packages Dr. Nicole Ferguson has authored a package called ”msSurv”, which provides the functionality necessary to do non-parametric estimation for multistate models. cran.r-project.org/web/packages/msSurv/index.html

9. Apply The apply function takes advantage of the “vectorized” nature of R. –R is fundamentally different from SAS. –SAS processes “records” within a dataset one a time. This makes SAS a very efficient user of memory but not of time. –R processes the entire data object R is fast but does not use memory efficiently.

10. Basic Functions We can substring text variables using the function substr(). More information can be found by typing ?substr at the R prompt. The function is –substr(variable, starts, stop) –Start is a numeric integer that represents the place to start sub-setting –Stop is where we should stop.

11. Examples Will be discussed in the R demo.